Explanation
The dots are white blood cells moving in the capillaries in front of the retina of the eye. Blue light (optimal wavelength: 430 nm) is absorbed by the red blood cells that fill the capillaries. The eye and brain "edit out" the shadow lines of the capillaries, partially by dark adaptation of the photoreceptors lying beneath the capillaries. The white blood cells, which are much rarer than the red ones and do not absorb blue light, create gaps in the blood column, and these gaps appear as bright dots. The gaps are elongated because a spherical leukocyte is too wide for the capillary. Red blood cells pile up behind the leukocyte, showing up like a dark tail. This behavior of the blood cells in the capillaries of the retina has been observed directly in human subjects by adaptive optics scanning laser ophthalmoscopy, a new real time imaging technique for examing retinal blood flow. The dots won’t appear at the very centre of the visual field, because there are no blood vessels there (foveal avascular zone).
Read more about this topic: Blue Field Entoptic Phenomenon
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